In today's business and scientific world, color is essential as a component of communication. Color facilitates the sharing of knowledge, and as a result companies involved in the development of digital color print engines are continuously seeking to improve the image quality of such products. One of the elements that affects image quality is the ability to produce accurate and consistent colors on a printer over time.
Color profiling, color calibration, and software/firmware manipulation of printer color output, for example a printer using a cyan, magenta, yellow, black (CMYK) color model (i.e., color pallet), is typically required to produce an accurate or desired color output. For profiling, the color to be profiled may be printed using a color step wedge or color gradient that represents a plurality of halftones, and ranges from an input of “0” which is intended to print no halftone dots over the area to be printed to an input of “255” (in an 8-bit system) that is intended to print a solid color over the print area. FIG. 1 is a graph 10 that represents a color tone response characteristic for a single color separation. If left uncorrected, a 0-255 color input will typically result an output that has an “S” shaped curve 12 that will produce incorrect colors over at least part of the 0-255 input when printed in combination with other colors. In one common method to correct or calibrate the color output, the halftone separation curve 12 is linearized by evaluating some or all of the halftone values from 0-255 and correcting output values to provide a linear color curve 14. For example, a digital input print value of 65 for a particular halftone may output an uncorrected digital output print value of, for example, 60 as depicted in FIG. 1. A digital translation is provided for each value in, for example, a lookup table. After profiling and correction, the digital input of 65 is corrected using the lookup table value to provide a corrected digital output print value of 65. Each color from a printer color pallet (e.g., CMYK) is separately evaluated to produce a different tone reproduction curve (TRC) for each color that linearizes the output for each color. The TRC for each color is stored in software or firmware and used during raster image processing (RIP) of the source material and printing of each color to result in a desired color output. The TRC thus corrects each halftone input from 0-255 with a customized output for each halftone for each color.
Colors on a printer tend to drift due to various factors such as color or viscosity variations in ink or toner, fluctuations in environmental conditions such as temperature and humidity, differences in the type of print media used, etc. When the drift is small, a print system may use a calibration procedure on each color halftone to correct the drift to achieve color consistency. This calibration to correct for small drift can be performed by re-calibrating each TRC for each color halftone.
For more significant drift where a larger separation interaction occurs, a new destination profile must be created for each halftone after calibration. To create a new destination profile, device-dependent color data from the printer is translated into a device-independent profile connection space (PCS) in accordance with international color consortium (ICC) specifications, for example using a lookup table. Translation may be performed by a color management module (CMM). Once the color values of the source device have been converted to PCS, the CMM may transform the color values from the PCS to a different device-dependent color space or destination profile, wherein each halftone includes a different destination profile.
Although profiling and calibration results in a high quality colorimetric print, running calibration and profiling multiple times leads to printer downtime and may be labor intensive in printers requiring manual calibration and profiling. For each color, the digital input must be compared with the digital output to create a new destination profile for each color. Further, it is difficult to track whether a specific halftone has up-to-date calibration before running a profiling procedure, particularly if the user intends to run only a subset of halftones. Some digital front ends (DFE) do not provide auto-switching mechanism and, in this case, the user is responsible for tracking which halftone is intended for the calibration and which profile should be associated therewith.
A simplified method for profiling and/or calibration of color printers would be desirable.